As terminals for displaying so-called images such as texts, still images, and moving images, CRTs (cathode-ray tubes) and LCDs (liquid crystal displays) have been widely used.
These devices are capable of instantly displaying and rewriting digital data, but it is hard for users to carry these devices with them at all times. In addition, since these devices are self-luminous, users get eye strain after a prolonged period of operations. Moreover, when these devices are turned off, display of images cannot be maintained. On the other hand, when texts and still images are distributed or stored in the form of a document, they are recorded on a paper medium by a printer. The paper medium has been widely used as a so-called hard copy. When users use the hard copy, they see reflections resulting from multiple scatterings.
Therefore, the paper medium is more excellent in visibility than the self-luminous devices, which reduces eye strain on users. In addition, since the paper medium is light and excellent in handling, users are allowed to read the paper medium in a comfortable position. However, the hard copy is discarded after use. Some hard copies are recycled, but recycling the hard copies requires enormous efforts and costs, which remains an issue in terms of resource saving. Recently, information processing such as document creation has been performed on a computer along with the advance of information equipment, and an opportunity for reading texts on a display terminal has been remarkably increased.
Under such circumstances, there has been a growing need for a paper-like display medium that has advantages of both the display and the hard copy, is rewritable, and is suitable for reading. Recently, attention has been paid to a polymer-dispersed liquid crystal, a bistable cholesteric liquid crystal, an electrochromic device, and a display medium using an electrophoretic device or the like as a display medium that can create reflective and bright display and have a memory property. Particularly, the display medium using the electrophoretic device is excellent in display quality and power consumption at a display operation, and various inventions related to the display medium have been disclosed.
For example, Patent Document 1 proposes a full-color reflective display. The full-color reflective display uses three electrophoretic sub-pixels, and each of the sub-pixels has a capsule including visually-different three types of particles. Furthermore, an individual electrode on the side opposite to a viewing side is composed of plural electrodes, the particles included in the sub-pixel are composed of particles having one through three types of hues, and a dispersion medium for dispersing the particles is transparent, dyed, or colored. Furthermore, the individual electrode is colored as occasion demands, or a color reflective plate is arranged at the rear surface of the transparent individual electrode. This display device is capable of expressing visual display in response to the application of an electric signal to at least one of the capsules.
Note that the content of the invention described in Patent Document 1 includes many things, but it has three particularly important configurations.
First, the first configuration is shown in FIGS. 3F through 3I of Patent Document 1. Taking FIG. 3F as an example, white particles W having a negative charging property and red particles R having a positive charging property are dispersed in a cell of a transparent dispersion medium. In addition, a common electrode 42 is arranged on the viewing side of the cell, and individual electrodes 35 and 45 are arranged at both ends of the cell on the side opposite to the viewing side of the cell. Furthermore, a substrate 60 colored in blue green is arranged beneath the individual electrodes. In this configuration, when a positive voltage is applied to the common electrode 42 and a negative voltage is applied to the individual electrodes 35 and 45, the white particles W move to the side of the common electrode 42, which in turn provides a white view on the viewing side (which corresponds to FIG. 3F of Patent Document 1). Furthermore, when a negative voltage is applied to the common electrode 42 and a positive voltage is applied to the individual electrodes 35 and 45, the red particles R move to the side of the common electrode 42, which in turn provides a red view on the viewing side (which corresponds to FIG. 3G of Patent Document 1). Furthermore, when a ground potential is applied to the common electrode 42, a positive voltage is applied to the individual electrode 35, and a negative voltage is applied to the individual electrode 45, the red particles R move to the side of the individual electrode 45 and the white particles W move to the individual electrode 35. In this case, since the individual electrodes are positioned at both ends of the cell, the white particles W and the red particles R do not move at the center of the cell. Therefore, the color of the substrate 60 is recognized on the viewing side (which corresponds to FIG. 3H of Patent Document 1). Furthermore, FIG. 3I of Patent Document 1 shows the arrangement of three types of the cells having different hues in this configuration. With this configuration, each of the cells can express a white color, the color of the colored particles, and the color of a substrate. However, it is clear from the above description that a black color cannot be expressed. Accordingly, the first configuration is not suitable for a display device that requires a black and white contrast such as text information.
Next, the second configuration is shown in FIG. 3L or FIG. 3M. In the configuration in FIG. 3L, the common electrode 42 is arranged on the viewing side of a region partitioned by the cell, and the individual electrodes 35 and 45 are arranged on the side opposite to the viewing side. The cell 22 contains a suspension fluid 62 colored in blue green in which the positively-charged red particles R and the negatively-charged white particles W are dispersed. In this configuration, the specification describes that the three basic hues recognized on the viewing side include a white state (a first state) in which the white particles W are positioned on the side of the common electrode, a red state (a second state) in which the red particles R are positioned on the side of the common electrode, and a blue green state (a third state) in which the red particles are positioned on the side of the individual electrode 45 and the white particles are positioned on the side of the individual electrode 35, thereby providing a view of the blue green suspension fluid on the viewing side. Furthermore, a configuration in FIG. 3M is composed of a transparent suspension fluid instead of the suspension fluid 62 in FIG. 3L and blue green neutral particles C, but the specification describes that the basic hues recognized on the viewing side are the same as those in FIG. 3L and the three basic hues exist.
When a full-color display is performed, three types of cells or micro-capsules having different colors of the particles and the suspension fluid are regularly arranged side by side with reference to FIG. 3J of Patent Document 1. More specifically, it is presumed that the first cell including the suspension fluid 62 colored in blue green, the positively-charged red particles R, and the negatively-charged white particles W; the second cell including the suspension fluid 62 colored in magenta, the positively-charged green particles G, and the negatively-charged white particles W; and a third cell including the suspension fluid 62 colored in yellow, the positively-charged blue particles B, and the negatively-charged white particles W, are regularly arranged side by side to perform a full-color display. However, in Patent Document 1, only the three types of the hues are capable of being displayed in one cell or one microcapsule. Therefore, even if the three types of the cells or the microcapsules are arranged side by side, an expressible color reproduction range is limited. For example, when a black text is displayed in a white background using the above three types of the cells, the configuration in FIG. 3L or FIG. 3M of Patent Document 1 does not have a hue corresponding to black. Therefore, a black display cannot be made.
Furthermore, if the negatively-charged black particles are used instead of the white particles W in the above configuration, the hue corresponding to black can be made. However, a white display cannot be made in this case. Note that the specification describes that when the suspension fluid colored in blue green or the blue neutral particles C are positioned in the cell or the microcapsule, the positively-charged red particles R are positioned near the individual electrode 45, and the negatively-charged white particles W are positioned near the individual electrode 35, the blue green particles are viewed on the side of the common electrode. This indicates that the suspension fluid or the blue green neutral particles absorb light other than blue green light and reflect the blue green light. Note that as described below, a colored dispersion medium according to an embodiment of the present invention is characterized to absorb light of a specific wavelength range and allow other light to pass through unlike the suspension fluid in Patent Document 1. Although one cell has only the three colors of the basic hues at maximum conventionally, the embodiment of the present invention is allowed to express four colors of basic hues according to this characteristic. This characteristic is an important difference between the embodiment of the present invention and Patent Document 1.
Furthermore, the third configuration is shown in FIG. 3E of Patent Document 1. In this configuration, each of the three sub-pixel capsules 22, 22′, and 22″ includes white particles 50, 50′, and 50″, respectively, dispersed in a transparent dispersion fluid. The sub-pixel capsules 22, 22′, and 22″ include transparent electrodes 42, 42′, and 42″ arranged on the viewing side; opaque electrodes 30, 30′, and 30″ arranged on the side opposite to the viewing side and having a small area; transparent electrodes 40, 40′, and 40″ arranged on the side opposite to the viewing side and having a large area; color filters 60, 60′, and 60″ arranged beneath the opaque and transparent electrodes; and a reflective substrate 70, respectively. The specification describes that this configuration has two types of basic hues in a first state in which the white particles are positioned on the opaque electrodes 30, 30′, and 30″ having the small area to express the color of the color filters and in a second state in which the white particles are positioned uniformly in the dispersion medium, positioned near the transparent electrodes 42, 42′, and 42″ arranged on the viewing side, or positioned near the transparent electrodes 40, 40′, and 40″ arranged on the side opposite to the viewing side and having the large area to express a white color. Accordingly, in this case also, the configuration does not have a hue corresponding to black. Therefore, a black display cannot be made. Furthermore, if the black particles are used instead of the white particles in the above configuration, the hue corresponding to black can be expressed. However, a white display cannot be made in this case.
Furthermore, Patent Document 2 proposes an electrophoretic device having an electro-optic layer between electrodes. This electro-optic layer includes a dispersion medium as an electro-optic liquid and particles contained in the dispersion medium. The particles are colored in a first color, and the dispersion medium is colored in a second color. The first and second colors are complementary to each other. According to this invention, when the dispersion particles are positioned near the electrode on the viewing side, the hue of the dispersion medium is recognized on the viewing side. On the other hand, when the dispersion particles are positioned near the electrode on the side opposite to the viewing side, light of a specific wavelength range incident from the viewing side is first absorbed by the dispersion medium. Since absorption of light by the dispersion medium and that of light by the dispersion particles are complementary to each other, light of a wavelength that has not been absorbed by the dispersion medium is absorbed by the dispersion particles. Accordingly, no light is reflected to the viewing side, which in turn provides a black view. In this configuration, a black display can be made. However, when it is desired that a white display be made, it is necessary to use the white dispersion particles and the black dispersion medium. Therefore, a color display cannot be made. Furthermore, in this configuration also, only the two types of the hues are capable of being displayed by one cell or one microcapsule. Therefore, even if the three types of the cells or the microcapsules are arranged side by side, an expressible color reproduction range is limited.
In other words, in the two related arts of Patent Documents 1 and 2, the configuration for displaying a white color cannot make a black or color display, and that for displaying a black color cannot make a white display. Accordingly, in the case of document information that requires black and white texts and color information, brightness in white and a black and white contrast are degraded.
Furthermore, Patent Document 3 proposes a method for filling an electrophoretic ink obtained by dispersing two types of particles having different charging properties and different colors in a translucent solvent, in a unit cell. A transparent common electrode is arranged on the viewing side, and laminated upper and lower individual electrodes are arranged on the side opposite to the viewing side. In addition, first and second opening parts each having no electrode are provided on the side opposite to the viewing side. The specification describes that with this configuration, a color absorbed by the two types of the particles, a color absorbed by the negatively-charged particles, a color absorbed by the positively-charged particles, and a color reflected by the individual electrodes can be expressed in one cell. However, this configuration is so complicated because the individual electrodes are vertically laminated to each other and the two opening parts are provided near the individual electrodes. Therefore, when it is assumed that the configuration is driven by TFTs, the structure of the TFTs becomes so complicated, which results in increased manufacturing costs.
Furthermore, Patent Document 4 proposes an electrophoretic display device that has display electrodes formed in multiple layers in such a manner as to laminate unit cells when each unit cell includes one display electrode or arrange them one on another toward an observer when each unit cell includes plural display electrodes. Colored electrophoretic particles that are electrophoretic and deposited on the display electrodes are translucent and form a display color by a subtractive color mixing principle for the observer. In this configuration, the two types of the cells having different hues are laminated to each other, thereby making it possible to express a full color using the subtractive color mixing principle. However, since this configuration requires the laminated structure of the display electrodes, it has specific disadvantages in practical use, such as (1) increased manufacturing costs due to a complicated device configuration, (2) interruption of light occurring when the TFTs are not transparent because the TFTs are required for each layer so as to drive the display device by the TFTs, and (3) a complicated display method because the transparent electrode on the viewing side is divided.
As described above, in the related arts, the three types of the hues can be expressed by one cell or one microcapsule at most. Therefore, even if the three types of the cells having different hues are arranged side by side, an expressible color reproduction range is limited.
Furthermore, in the case of the configuration in which the two types of the cells are laminated to each other and the configuration in which the individual electrodes are laminated to each other, the structure of the display device becomes complicated. Furthermore, the configuration in which the two types of the cells are laminated to each other is not suitable for driving the TFTs.
The present invention has been made in view of the problems residing in the above related arts and may provide an electrophoretic liquid that increases the number of basic hues that can be expressed by one type of liquid and expands an expressible color reproduction range.
Also, the present invention may provide a display device that realizes both brightness and an expanded color reproduction range with a simple configuration as a reflective display device using the electrophoretic liquid. Moreover, the present invention may provide a display device using an electrophoretic liquid that is capable of expressing a bright white color and a high density black color and suitable for displaying texts even when a color display is made using a method for arranging three types of cells having different hues, and may provide a reflective display device that has an expanded color reproduction range compared with a conventional case in which three types of cells having different hues are arranged side by side even when a color display is made.
In a separately-filed application JP-A-2009-9092, the applicant has proposed an electrophoretic liquid (hereinafter referred to as an invention A) in which three types of dispersion particles having different optical properties and charging properties are dispersed in a solvent. In the electrophoretic liquid, the first dispersion particles have no charge, the second dispersion particles are positively-charged electrophoretic particles, and the third dispersion particles are negatively-charged electrophoretic particles. Specifically, the first dispersion particles having no charge are colored in white, the second positively-charged dispersion particles are colored in any of cyan, yellow, and magenta, and the third negatively-charged dispersion particles are colored in black. When a full-color display is made, three types of cells having different colors of the second dispersion particles are regularly arranged side by side to combine the hues of the three types of the cells with each other. Thus, a desired color can be expressed. This invention can express not only the hues of a white display and a black display but also a color hue unlike Patent Documents 1 and 2. However, in the invention A, the three types of the hues are capable of being displayed in one cell. Therefore, even if the three types of the cells or the microcapsules are arranged side by side, an expressible color reproduction range is limited. For example, when a red hue is expressed, red dispersion particles do not originally exist. Therefore, the red hue is expressed when the yellow particles (that absorb light of a wavelength of 400 nm through 500 nm) and the magenta particles (that absorb light of a wavelength of 500 nm through 600 nm), which absorb part of light of a wavelength of 400 nm through 600 nm for absorbing red light, are positioned at the electrode on the viewing side.
However, since it is desired that light of a wavelength for absorbing red be ideally absorbed by all the three types of the cells, only an extremely light red color can be expressed. Similarly, when a green color and a blue color are expressed, only an extremely light green color and an extremely light blue color can be expressed. On the other hand, when a magenta hue is expressed, a cell including the dispersion particles having the magenta hue expresses the magenta hue, but a cell including the dispersion particles having the yellow hue and a cell including the dispersion particles having the cyan hue cannot express the magenta hue. Therefore, a white or black color must be expressed. Accordingly, an extremely light magenta or a blackened magenta is necessarily expressed as a whole. Similarly, when a cyan color and a yellow color are expressed, only an extremely light cyan color, an extremely light yellow color, a blackened cyan color, and a blackened yellow color can be expressed.    Patent Document 1: JP-A-2002-511607    Patent Document 2: JP-A-2001-290444    Patent Document 3: JP-A-2007-310182    Patent Document 4: JP-A-2004-20818